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168                         METALLURGY OF IRON AND STEEL.
flame is longer on account of there heing double work to do before the combustion is complete. Each molecule of oil, as it comes into a hot furnace, undergoes a process of dissociation, the rich hydrocarbons breaking up under the tension of internal molecular activity. This absorbs heat, and for an instant the disruption lowers the temperature below the point of ignition. Moreover, as each point of oil explodes, it makes a small balloon of gas, and it takes a moment for this to become mixed with the.air necessary for its combustion. If steam is present its reduction by carbon entails a! certain delay.
These matters may seem trifling., but they are probably the explanation of the very important fact that, under the usual conditions of furnace operation, a flame from oil vapor is longer than a flame from coal gas. In the burning of clear carbonic oxide, or a mixture of it with nitrogen, there is no preliminary decomposition' to be performed, the air being free to immediately touch and burn the molecules of the fuel.
It is impossible to state the comparative economy in the use of coal and oil, since their relative values vary so widely in different localities, but it may be assumed that 50 gallons of oil are equivalent to 1000 pounds of soft coal in regenerative furnaces or under boilers. '
(c)  Water gas:
NOTE : This discussion is condensed from an article by George Lunge, in The Mineral Industry for 1901.
When steam is passed over incandescent carbon the subjoined reaction takes place: .
C+H20=rCO+H2.
' Equal volumes of carbon monoxide and hydrogen are formed, the mixture possessing the caloric value of 2800 metric heat units per cu. m., an amount one-half the heat value of gas made by distilling bituminous coal in retorts. The heat produced by gram-molecules is for CO+H2+02=C02+H20=^68.4+57.6=l26 heat units, whereas the direct combustion of carbon, C-}-02=C02, produces only 97 heat units. The introduction of water cannot be the source of energy, and the apparent gain of 12697=29 heat units must come from the heat that accumulates in the incandescent fuel.
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